4 ohm or 16 ohm?

hipofutura

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In Example 1, yes, you get 30W per speaker for a total of 60W delivered to your load. However, this will NOT sound the same as going to 1 8 ohm speaker, because of the way your ears and brain perceive sound. I'm not an expert on that, but you can just plug your amp into 2 different cabs at the store and see what I'm talking about.

In Example 2, I don't feel like doing the math when I'm not getting paid for it! Plus, I would have to know the true impedance of your source (amplifier) and load (speakers), not just the DC resistance. So I don't know what the answer will be. However, power transfer theorems say that it MUST be less total power delivered to your load than if it was a matched load. How much less depends on the math that we don't have all the information to do. Obviously, the more mismatch in impedance the less total power delivered.


I need a bigger brain to keep up with you guys!
 

The Mick2

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To my ears with my JMP 4 ohms just sounds better I don't know why it just does.
 

ErictheRed

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Here's a good little tutorial that might help: Maximum Power Transfer Theorem.

What I don't understand is why some people care about this stuff. In other words, why bother NOT matching your impedance? Is it just idle curiosity? If that's the reason, go the guitar store and plug the same amp into multiple cabs of different impedances and see if you notice any difference* (I'm guessing you won't unless you start playing loud). Is it that people are too lazy to wire their speakers correctly or buy the right kind of speakers to begin with?

Just match impedances and be done with it; you'll never have to worry about anything going wrong and you'll never have to think about power transfer ever again if you don't want to. I only think about it because it's my job to.



*This is potentially dangerous, however I highly doubt that any damage will occur unless the mismatch is much bigger than the typical 4 ohm to 16 ohm difference. But you never know...
 

Reeko

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One thing to realize is that it is different for SS amps vs Tube.

Tube amps I believe must have a load on them or you damage the output section. I believe on tube amps you are supposed to err on the lower than rated load rather than the higher

For example, if rated at 8ohm (tube amp) and you only have the option of running 4ohm or 16 ohm, it is safer to run 4 ohm. Running too high a load cxan damage the amp.

SS amps are the opposite, you can run them with nothing attached (infinate load) but too low a load will damage them.

(BTW: I am also an electrical enginerr, but it has been years since I did any analog stuff.)
 

ErictheRed

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The only difference between SS and tube amps, when it comes to this, is that tube amps have an output transformer. But, SS amps still have some output impedance (their driver circuit) that should be matched to the load as well.

Reeko, I'm not certain that running 4 ohms would be safer than running 16. I've heard people claim the opposite as well. I've never gotten involved because I've never wanted to do the real math or circuit modelling that would be required to answer that (though I'm sure some amp manufacturer has already done it). Plus, it will vary depending on the design of the individual amp. So I don't think it's helpful to spread what is, at least sometimes, misinformation. I don't want anyone thinking "Well, as long as I use a smaller load, it will be safe."

By the way, I do RF design and test, so at least in principle, I deal with this stuff daily. In reality the circuits I deal with are much different than guitar amp circuits, but the principles are all the same.

Yuck I'm tired of talking about technical stuff on a guitar website...
 

Reeko

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I got the following from a Web FAQ. at Tube Amp FAQ 1/20/99 (Frames supported)

This is what I always heard about matching Tube Amp loads...


Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.
 

ErictheRed

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I actually agree with this about flyback voltages, but worrying about the output transformer is only one area of concern. Their are also resonance issues, potential stability problems, life of power tubes, whether or not an amp is class A or A/B...here are a few problems associated with too low a load:

With too small a load, the primary side of the OT presents a low load to the tube. The triode's plate can be considered a voltage source, and it will try to source tons of current to make the plate voltage swing. If the load was a dead short, it couldn't do this, so the cathode will burn up (overheat). A pentode is more of a current source and will continue to pass the plate current into whatever lower impedance it sees.

Aaahhh anyway I really don't care anymore, I'm not going to waste my time trying to type up stuff that can be found in dozens of engineering text books. You absolutely can NOT say that a mismatch in one direction is safer than another without knowing a lot about the individual amp's design.

I think this is the last I will say on this topic, I don't mean to be pissy, I just don't want to think about this stuff when I'm not at work. I try to share my technical knowledge to help people, but I don't have time to debunk all the misinformation all over the web. I made that mistake in a thread about Evolution vs. Creationism once (different website) and I won't do it again.
 

SteveGangi

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I just match the impedances, so I won't have to "bother with it" later on. It's easy enough to come up with 4, or 8, or 16 Ohms through the appropriate wiring and soldering. And, as the other guys said,. the math is just too much bother with for now. Just keep it simple. match the load impedances, and you won't have to worry about what circuits will burn up from what type of mismatches.
 

BillB1960

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OK so I have a tube amp head that has a rated output of 12 ohms...not kidding...true story. I've personally never seen a 12 ohm speaker or cab. So do I use an 8 ohm cab or a 16 ohm cab? Either way is a mismatch...up or down? Why would an amp builder purposely make a 12 ohm output?

The answer BTW according to the builder is: Use either an 8 or a 16, just not a 4 or a 2. A 4 ohm mismatch is not going to be that big of a deal either way.
 

Reeko

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I have heard that most amps can stand a 2X mismatch OK, but can't confirm.
So A 12ohm Head into 8 or 16 ohms should be OK.

Also, you could go with 6 speakers (a 4x12 and a 2x12 cab perhaps).
Then for example, hook pairs of 8ohm speakers in parallel (resulting in 4ohms each). Then run those pairs in series giving 12 ohms.
 

Lester

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Thanks for all the info. To answer Eric's question about "why even bother... just match them", it comes down to a case of amps with no load setting switch, speakers installed, speakers I already have that I'd like to use, and a potentially nice deal in the marketplace. Add it all together, and I end up with questions like "it says 8 ohm min, what happens if I run it 16 ohms?" Anyway, thank's for the info.

As for the Evolution vs. Creationism, we'll that's a little tougher. One interesting related story is that of Clapton, who never claimed divinity, but had it thrust on him by others (e.g. "Clapton is God"). Seems to parallel some other story I've heard, but I can't remember which :)
 

ErictheRed

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Just wondering, can I use the same method as Wiring a 4 X 12 Speaker cabinet using the third example to create 8ohm 2x12 cabinet with two 8ohm speakers?:hmm:

I'm not really sure I get what you're asking. But no, you cannot wire 2 8 ohm speakers to get a total of 8 ohms. You can put them in series for 16 ohms or parallel for 4 ohms, but that's it. You could short one of them out and get 8 ohms, but then you're only using 1 speaker :).

You can make connections that are neither series nor parallel, like delta or wye connections. That might be an option to handle a weird number of speakers (like 3 or 6) but I've never heard of anyone doing it.
 

Nicky

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I have a TSL-122. The two Celestion speakers in the combo are 8Ω wired in series = 16Ω. I have a Marshall 2x12 cab with two Celestion 8Ω speakers, but it was orginally wired parallel, so I rewired as series = 16Ω. I then set the TSL impedance switch to 8Ω, then plugged the combo speakers into one 8Ω parallel jack, and the cab into the other 8Ω parallel jack. So, now I have two 16Ω loads in parallel to match the combo amp's rated impedence per channel of 8Ω.

Is this correct?
 

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